Four artificial lift systems commonly used in deep wells of low output are: beam pump systems; gas lift; submergible centrifugal pump systems; and hydraulic pump systems.
Major components of a beam, or sucker rod, pump system are a surface mounted reciprocating beam unit, a sucker rod string, and a downhole pump. The surface unit is the largest expense item in the system, and the expense increases as the pumping depth and flow rate increases. A considerable amount of preventive maintenance is required in order to maintain the system operational, since the moving parts need lubrication, and the system includes a stuffing box requiring periodic tightening to ensure good sealing of the sucker rod at the surface. Also, with this system, as the rod string increases in length, it tends to stretch and contract during a greater portion of the stroke, thus reducing the efficiency of the system.
Gas lift is a relatively simple and reliable method for obtaining lift in a well. It is accomplished by injecting gas into the well at predetermined depth(s), either continuously, to lower the pressure of the formation so that the fluid will flow freely, or intermittently, at a high instantaneous rate for a short time, to surface columns of fluid at regular intervals. There is, however, a sharp increase in power required per barrel of fluid produced at rates below 250 barrels per day, although this is influenced, to an extent, by tubing size. The gas lift technique also requires a source of gas which may not be present at a well in the required quantities.
Submergible centrifugal pumping systems generally consist of an electric motor, a motor protector, a centrifugal pump (usually having multiple stages) and frequently a gas separator at the pump intake. Traditionally, these systems have been best applied in wells of high output (in excess of 250 BPD) at depths up to 15,000 feet. Centrifugal pumps tend to become inefficient at low flow rates, and difficult to produce when the openings in the stages become narrow slits designed to pump at low flow rates.
Hydraulic oil well pumping systems generally consist of a surface pump and filtration system which supplies high pressure clean fluid to a downhole hydraulically driven reciprocating pump. With these systems, high pressure power fluid at the surface can present a fire hazard in case of a leak. Further, in many cases, the system requires an extra tubing string for the flow of well fluid to the surface, and this can be a major cost item. Due to the separation of the donwhole pump from the surface fluid supply, the amount of energy stored in compression of the fluid and expansion of the supply line can be considerable. If the downhole pump control valve is not designed to handle this fluid in a no-load condition, the pump may reciprocate too rapidly, resulting in premature failure. This necessitates a complex screening valve.
The present invention provides alternative downhole pumping apparatus that is more satisfactory than traditional systems, particularly in deep wells (for example, deeper than 5,000 feet) that have low output, i.e., lower than 250 barrels per day.